The invention pertains to an indium oxide/tin oxide sputtering target for cathodic sputtering with 5-15 wt. % of tin oxide and with a density of at least 95% of the theoretical (maximum) density and to a process for the production of these targets by hot isostatic pressing.
Oxide ceramic targets of indium oxide-tin oxide (ITO) are used for the production of thin, transparent, electrically conductive layers by cathodic sputtering. Layers such as these are used primarily in flat image screen technology. The thin indium oxide/tin oxide layers can be produced either by the sputtering of metal targets in a reactive atmosphere containing oxygen or by the sputtering or oxide ceramic targets. The sputtering process with oxide ceramic targets offers the advantage that, because there is only a weak flow of oxygen in the sputtering chamber, it is easier to regulate the sputtering process than it is when sputtering is carried out at the high oxygen flows required for sputtering with metal targets.
Because indium oxide/tin oxide is a ceramic material which has much lower thermal conductivity than metals and which at the same time is highly sensitive to the thermal stress resulting from uneven heating during the sputtering process, there is great interest in being able to produce targets with good thermal conductivity. These targets would then make it possible to carry out the sputtering operation at a higher specific power density without running the risk that the targets will fail as a result of cracking or fracturing. If it is possible to operate at a higher sputtering rate, production can be carried out with a higher substrate throughput, which increases the capacity of the system and lowers production costs. In addition, a target with better thermal conductivity means that, if the sputtering rate is not increased, the temperature at the surface of the target will be lower. The reduced heat gradient reduces the thermal stresses in the target material and therefore reduces the likelihood of target failure and increases the reliability of sputtering operations. This is especially true when the process is carried out with cathodes which are operated in a pulsed manner.
Systematic studies were therefore conducted to discover a set of conditions under which it would be possible to produce targets consisting of mixtures of indium oxide and tin oxide containing 5-15 wt. % tin oxide which have significantly better thermal conductivity than that of conventional targets. The targets currently obtainable on the market have a thermal conductivity of no more than 10 Wm.sup.-1 K.sup.-1. As a rule, values of only 7-9 Wm.sup.-1 K.sup.-1 are reached.
Preliminary experiments showed that a basic condition for high thermal conductivity is the highest possible density of the target material, i.e., a density of .gtoreq.95% of the theoretical density. This alone, however, did not prove to be enough. Through variation of the powder treatment and powder compaction steps, an attempt was made to find conditions under which not only a high target density but also a high thermal conductivity would be obtained. It was found that the processes according to the state of the art described did not lead to good results.
Partially reduced indium oxide/tin oxide targets and methods for their production are described in U.S. Pat. No. 4,690,745 (Klein). According to this patent, indium oxide/tin oxide powder mixtures are hot-pressed under reducing conditions at 850-1,000.degree. C., the oxides being hot-pressed in a graphite hot-press mold or under the addition of carbon or carbon-releasing organic material. By pressing under these conditions, the oxides are partially reduced, so that a target is formed with a slightly depleted oxygen content, i.e., an oxygen content slightly below that of the stoichiometric composition.
By means of this process, targets with densities of only up to 91% of the theoretical density (TD), with an unsatisfactory thermal conductivity of .gtoreq.8 Wm.sup.-1 K.sup.-1, and with poor electrical conductivity (p=0.1-0.6 .OMEGA.-cm) are produced.
Processes for the production of partially reduced ITO targets by the use of pre-reduced powder and subsequent hot-pressing or hot isostatic pressing are described in DE 41 24 471 (Weigert) and in DE 44 07 774 (Schlott). Studies on the influence of the degree of reduction of the ITO powder on the thermal conductivity of the target material have shown that, on average, a higher thermal conductivity is obtained when the targets are partially reduced. On the basis of the processes described, however, it was not possible to arrive at targets with thermal conductivities of more than 11 Wm.sup.-1 K.sup.-1.
Processes for the production of ITO targets by sintering have been described by various authors (U.S. Pat. No. 5,071,800 (Tosoh Corp., JP 4-1293707 (Dowa)). Targets produced according to the processes described are not usually dense enough (.ltoreq.90 TD). The thermal conductivities reached in these cases are only around .ltoreq.9 Wm.sup.-1 K.sup.-1.
The task of the present invention is therefore to avoid the disadvantages of the known target materials with respect to insufficient thermal conductivity and to create an oxide ceramic target with high thermal conductivity.